Electric discharge lamp with phosphorcoated rhodium reflector



March 12, 1957 .1. T. ANDERSON 2,785,327

ELECTRIC DISCHARGE LAMP WITH PHOSFHOR-COATED RHODIUM REFLECTOR Filed Feb. 17, 1954 6 (PHosPHoR) 5(RHoo1uM) 4(Su vER on ALUMINUM) inventor: James T. Anderson, by MK His Attorney United States Patent ELECTRIC DISCHARGE LAMP WITH'PHOSPHOR- COATED RHODIUM REFLECTOR- J m T-Andersw. Hi lmotton,Ru hmE g a assienor to General Electric Company, a corporation of New York a i Application February 17, 1954, Serial No. 410,850

Claris- (c This invention relates to. electric discharge lamps in which a luminescent material is located in contact with a metal deposited. to forma reflecting surface upon which light from. the discharge impinges and from which it. is reflected.

It has been suggested to improve the light utilization and color renderingof. an electric discharge lamp of the high pressure mercury. vapor type by mounting it in an outer glass bulb so shaped that when a part of the interior surface of the bulb is coated with a reflecting material, suchas silYeLQr. aluminum, it, forms a reflector whichre istri utes he. i h m tte fr m the mp, the reflector being coated with a luminescent material or phosphor which is excited by the radiation from the discharge to emit light capable of improving the color of the lamp.

It has been found, with certain phosphors, particularly those containing large amounts of magnesia and/ or highly oxidized manganese, or other elements, that the metal, silver, usually employed to form the reflecting surface is attacked with damage to the reflector and to the phosphor. Aluminum, sometimes used as an alternative for silver, is also attacked by some phosphors, although usually to a lesser degree.

This disadvantage is removed, according to the invention, by using, as the coating producing the surface of high reflectivity, a film of rhodium. By this means we have been able to retain a high degree of reflectivity and eliminate the reaction between the phosphor and the reflector.

The rhodium may be applied by any of the methods employed for producing reflecting coatings of silver, e. g., thermal evaporation in vacuo, electrodeposition, or the like. If evaporation in vacuo is employed and a prior coating of silver or aluminum is used beneath the rhodium, the two metals may be vaporized one after the other without releasing the vacuum.

In the single figure of the drawing accompanying and forming part of this specification an electric lamp embodying the invention is shown schematically with a part thereof broken away and shown in section.

Referring to the drawing, the lamp comprises an arc tube I mounted in a sealed glass envelope 2 provided with a screw type base 3. The envelope 2 has on its inner surface around the arc tube 1 a metalized reflector 4, 5. The lamp may be of the type disclosed and claimed in U. S. Patent 2,491,868, granted December 20, 1949, upon an application of Ernest Martt, and which is assigned to the assignee of the present application.

In accordance with the present invention the reflector 4, 5 is made up of a layer 4 of silver or aluminum deposited on the inner surface of the glass bulb 2 and a layer 5 of rhodium deposited on the layer 4 and constituting the reflecting surface of the reflector 4, 5. A coating 6 of luminescent material of the kind excited to visible light emission by radiation from the are tube 1 covers the rhodium layer 5. It will be understood, of course, that the thickness of the layers 4, 5 and the coattration.

Suitable luminescent. materials or phosphors for use with. such a rhodium surface reflector in a lamp of the type described are. those containing magnesia, manganese or. other materials. reactive with the usual metals, silver and aluminum, used heretofore as reflective coatings for lamps of this type. Examples of such phosphors are the following: manganese-activated magnesium fluo-germanate, cerium and manganese-activated calcium orthophosphate. and manganese-activated magnesium arsenate.

While a layer 4 of silver or aluminum has been shown in the drawing directly on the inner surface of the envelope 2 and coveredby the rhodium layer 5, it will be understood, of course, that "the silver or aluminum coating. 4 may be. omitted and the rhodium layer 5 used alone as. the reflecting material and applied directly to the inner surface of the glass envelope.

The. coated portion of the envelope 2 surrounding the arc tube 1 is of paraboloidal contour and the lower, light transmitting part 7 thereof may be transparent or translucent.

The are tube 1 may be of the well-known commercial type comprising a sealed tubular quartz envelope having two, main. discharge supporting electrodes mounted at its. ends and anauxiliary. electrode mounted adjacent one of the main electrodes for starting purposes. The tube contains a starting gas at a few millimeters pressure and mercury in an amount suflicient to provide a high pressure mercury vapor atmosphere, but limited so as to be completely vaporized at a temperature slightly below the elevated temperature at which the arc tube is designed to operate.

The outer envelope 2 protects the arc tube from drafts or the like during operation and encloses the inner electrically conducting parts of the lamp. The envelope 2 may be filled with a gas such as nitrogen at a reduced pressure.

Such lamps are well-known commercially, and since the internal supporting structure for the arc tube and the means for supplying electrical energy thereto constitute no part of the present invention, these internal parts of the lamp have been omitted for simplicity of illustration of the invention.

An electric discharge lamp having the structure described above and shown in the drawing has a substantially longer useful life because of its better maintenance of light from the phosphor and retention of reflectivity by the reflector than prior lamps of the same structure but omitting the layer of rhodium utilized as the reflecting and phosphor supporting surface of the new lamp.

This invention may also be employed in cases where the reflector and phosphor are not contained within the lamp envelope 2 itself but are mounted outside the lamp and in light receiving relation thereto. Other modifications will readily occur to those skilled in the art. Such variations are to be considered within the purview of the specification and the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge lamp comprising a vitreous envelope containing ultraviolet generating means, a refleeting coating in contact with the inner surface of said envelope, the inner surface of said reflecting coating comprising a layer of rhodium, a phosphor coating in contact with said layer of rhodium, said phosphor coating being excitable to light emission by radiation from said generating means and containing material chemically reactive with the usual reflective coating materials, silver and aluminum, and inert with respect to rhodium.

2. A high pressure mercury vapor discharge lamp comprising a quartz are tube mounted within a glass envelope, a reflecting coating in contact with the inner surface of said envelope, the inner surface of said refleeting coating comprising a layer of rhodium, said reflecting coating facing said are tube, and a coating of phosphor in contact with said layer of rhodium, said phosphor coating being excitable to light emission by radiation from said arc' tube and containing material chemically reactive with the usual reflective coating materials, silver and aluminum and inert with respect to rhodium. W

V 3. A high pressure mercury vapor discharge lamp comprising a quartz arc tube mounted Within a glass envelope, a reflective metal coating in contact with the inner surface of said envelope, said reflective coating consisting of a first layer of a metal of the group consisting of silver and aluminum deposited directly on the inner surface of the said glass envelope and a Second layer of rhodium in contact with said first layer, said phosphor coating being in contact with a rhodium layer and being excitable to visible light emission by incident radiation from said are tube and containing material chemically reactive with the metal of the first layer and inert with respect to rhodium.

4. A high pressure mercury vapor discharge lamp comprising a quartz arc tube mounted Within a glass envelope, a reflecting metal coating in contact with the inner surface of said envelope, said reflecting coating consisting of a first layer of a metal of the group consisting of silver and aluminum deposited directly on the inner surface of the said glass envelope and a second layer of rhodium in contact with said first layer, and a coating of phosphor in contact with said rhodium layer and being excitable to visible light emission by incident radiation from said are tube and containing a material of the group consisting of magnesia, oxidized manganese and mixtures thereof.

5. In combination, a high pressure mercury vapor discharge lamp, a reflector mounted in light receiving relation to the luminous discharge in said lamp, said reflector including a reflecting surface comprising a layer of rhodium,-and a phosphor coating in contact with said layer of rhodium, said phosphor containing material reactive with silver and aluminum and inert to rhodium.

References Cited in the file of this patent UNITED STATES PATENTS 2,304,182 Lang Dec. 8, 1942 2,305,050 Wise et a1. Dec. 15, 1942 2,476,681 Overbeek et a1. July 19, 1949 2,491,868 Martt Dec. 20, 1949 2,509,071 Pennybacker May 23, 1950 2,519,545 Colbert et al Aug. 22, 1950 FOREIGN PATENTS 599,678 Great Britain Mar. 18, 1948 631,259 Great Britain Oct. 31, 1949 All, 

1. AN ELECTRIC DISCHARGE LAMP COMPRISING A VITREOUS ENVELOPE CONTAINING ULTRAVIOLET GENERATING MEANS, A REFLECTING COATING IN CONTACT WITH THE INNER SURFACE OF SAID ENVELOPE, THE INNER SURFACE OF SAID REFLECTING COATING COMPRISING A LAYER OF RHODIUM, A PHOSPHOR COATING IN CONTACT WITH SAID LAYER OF RHODIUM, SAID PHOSPHOR COATING BEING EXCITABLE TO LIGHT EMISSION BY RADIATION FROM SAID GENERATING MEANS AND CONTAINING MATERIAL CHEMICALLY REACTIVE WITH THE USUAL REFLECTIVE COATING MATERIALS, SILVER AND ALUMINUM, AND INERT WITH RESPECT TO RHODIUM. 